The present invention relates to a fuel injection control method for an internal combustion engine having an electronic control system such as a microcomputer.
The fuel injection control system having the microcomputer is widely used in various types of engines such as a four-cycle engine and a two-cycle engine.
Japanese Patent Application Laid-Open 2-108827 discloses such an electronic fuel injection control system for the two-cycle engine. In the system, operating frequency of a fuel injector is controlled based on crankcase inner pressure and engine speed, or throttle valve opening degree in dependency on the engine operating conditions.
An applicant of the present invention have proposed a fuel injection control system for the two-cycle engine disclosed in Japanese Patent Application Laid-Open 3-175121 and 3-175131. In the systems, fuel injection quantity is controlled in accordance with various engine operating conditions such as engine speed and throttle opening degree as parameters.
In the prior art, the fuel injection is controlled by the open-loop method. If the fuel pressure deviates out of a normal range in such an abnormal condition that the amount of fuel in the fuel tank is very small or a pressure regulator does not properly operate, the air-fuel mixture becomes extremely rich or extremely lean. However, the open-loop control system can not correct such an unusual air-fuel mixture. Furthermore, if the air-fuel mixture becomes extremely rich, the ignitability of the fuel reduces, which causes unstable combustion operation of the engine. In particular, in the two-cycle engine, combustion condition affects the temperature of the crankcase and cylinders. If a lean air-fuel mixture continues for a long time, the engine operation becomes unstable. Therefore, it is necessary to prevent the air-fuel mixture from becoming extremely lean.
Japanese Patent Application Laid-Open 2-95747 discloses a system for controlling the air-fuel ratio by feedback control. In the system, an O.sub.2 -sensor is provided in the exhaust pipe for detecting an extreme lean air-fuel mixture caused by percolation in a fuel injector at a high temperature of the engine and for detecting an extreme rich air-fuel mixture caused by high intake air temperature in order to control the air-fuel ratio.
If such a system is employed in the two-cycle engine, the air-fuel ratio is properly controlled in dependency on the engine operating conditions.
However, a time elapses before the air-fuel ratio is properly controlled because of control delay of the feedback control system. Therefore, undesirable engine operation continues for a while.